The present invention relates to an interference canceller using an array antenna for a Direct Sequence Code Division Multiple Access (DS-CDMA) system and a radio communication device using such an interference canceller.
In the DS-CDMA system, a multi-rate transmission system including a plurality of transmission rates is known. Since the channel capacity is affected by interference between channels, it is necessary to provide an interference canceller for eliminating the interference between channels. A structure is known in which transmission power is reduced by using an array antenna to form a beam. The use of the array antenna results in a reduction in the interference due to spatial separation, which provides an improvement in the antenna gain. A RAKE receiver system capable of coping with multi-paths is also known. It is considered that the above-mentioned various techniques may arbitrarily be combined. In this case, it enables improved characteristics with an appropriate cost.
A transmitter part in a mobile radio communication system generally includes an encoder part performing error correction and encoding, a modulator part using a given modulation system such as QPSK, and a spread demodulator part. A receiver part which corresponds to the above transmitter part generally includes a spread demodulator part sing despreading, a modulator for demodulating a modulated wave of the QPSK or the like, and a decoder part performing error correction.
FIG. 10 is a diagram showing a conventional interference canceller. As can be seen, the interference canceller has a multi-stage parallel structure including a first stage, a second stage and a final stage. In this figure, 101 indicates an antenna, 102-1, 102-2 indicate delay circuits (DL), 103-11-103-1k, 103-21-103-2k indicate interference replicating units (IRU), 104-1,104-2 indicate adders, and 105-1-105-k indicate receivers corresponding to different users. In the conventional interference canceller, it is possible to use only one stage or a number of stages.
In the first stage, a received signal is applied to the interference replicating units 103-11-103-1k corresponding to the different user channels. The units 103-11-103-1k then output symbol replicas SB and interference replicas d. The adder 104-1 subtracts the interference replicas d from the received signal obtained through the delay circuit 102-1.
A resultant signal is applied to the second stage as an error signal e.
In the second stage, the interference replicating units 103-21-103-2k units are provided with the error signal e from the first stage and the symbol replicas SB from the interference replica creating units 103-11-103-1k. The interferencing cancelling units 103-21-103-2k then output symbol replicas SB′ and interference replicas d. The adder 104-2 then subtracts the interference replica d′ from the error signal e supplied from the first stage via the delay circuit 102-2. A resultant signal is applied, as an error signal e′ to the receivers 105-1-105-k of the final stage. The receivers 105-1-105-k which correspond to the different user channels perform a demodulation process by using the error signals e′ and the symbol replica SB′ from the second stage. Therefore, the received signal is obtained in each of the receivers 105-1-105-k.
FIG. 11 is a diagram showing a conventional interference replicating unit. The reference number 103 indicates the interference replicating unit shown in FIG. 10. In FIG. 11, 111 indicates despread processing parts, 112 indicates respread processing parts, 113 indicates a despreading part, 114 indicates an adder, 115 indicates a channel estimation part, 116 indicates a multiplier, 117, 119 denote combining parts (Z), 118 indicates a decision part, 120 indicates a multiplier, 121 indicates an adder, and 122 indicates a respreading part.
The despread processing parts 111 and the respread processing parts 112 are provided in a parallel form taking into account that a signal is received through a plurality of propagation routes caused by reflection or the like. Hence, the parts 111, 112 are equal in number to the paths of the propagation routes. The despread processing parts 111 are each provided with a received signal, or the error signal e and the symbol replicas SB from the previous stage (the symbol replicas SB of the first stage are zero). In each of the despread processing parts 111, the despreading part 113 despreads the received signal or the error signal e by a spreading code so that the demodulated signal can be obtained. The adder 114 then adds the symbol replica SB of the previous stage to the demodulated signal. The resultant output signal of the adder 114 is applied to the channel estimation part 115 and the multiplier 116. The channel estimation part 115 applies a channel estimation value to the multipliers 116, 120.
In this case, the received symbol is represented as Z·ξ where the known symbol such as a pilot signal is denoted as Z (complex number) and the propagation characteristic of the path is denoted as ξ (complex number). Thus, by multiplying the received symbol Z·ξ by the complex conjugate of the known symbol Z*, |Z|2·ξ can be obtained. As described previously, since the symbol Z is known, the propagation characteristic of the path ξ is obtained. Therefore, the average value of the propagation characteristic t can be handled as a channel estimation value ξ{circumflex over (0)}.
The complex conjugate (indicated by symbol *) of the channel estimation value ξ{circumflex over (0 )}. is input to the multiplier 116, which multiplies the output signal of the adder 114 by the channel estimation value ξ{circumflex over (0)}. The resultant output signals of the despread processing parts 111 are summed up by the combining part 117, so that a path-diversity synthesized signal is obtained. In the path-diversity synthesized signal, the phase differences resulting from the propagation paths are already corrected.
The decision part 118 compares the synthesized output signal of the combining part 117 with a threshold value and the outputs provisionally decided data. In each of the respread processing parts 112, the multiplier 120 multiplies the decision output signal of the decision part 118 by the channel estimation value output by the corresponding channel estimation part 115 is which is output to the next stage as a symbol replica SB of the corresponding path. The adder 121 calculates the difference between the symbol replica of the present stage and the symbol replica SB of the previous stage. The difference is respread by the spreading code by the respreading part 122. The combining part 119 sums up the output signals of the respreading parts 122 corresponding to the respective paths and than outputs a resultant interference replica d, which is sent to the next stage.
The receivers 105-1-105-k of the final stage shown in FIG. 10 are configured by modifying the interference replicating unit 103 of FIG. 11 so that the respread processing parts 112 are omitted and a demodulator is provided for modulating the resulting signal. The demodulated signal is sent to a network (not shown) in which a base station is connected.
FIGS. 12A and 12B show an adaptive array receiver device. In particular, FIG. 12A shows a receiver device made up of array antenna elements 131-1-131-m and adaptive array receivers 132-1-132-k (AA receivers) corresponding to the different respective users. FIG. 12B shows the configuration for each of the adaptive array receivers 132-1132-k. In FIG. 12B, reference number 133 indicates despread processing parts, 134-1-134-m indicate despreading part, 135-1-135-m indicate multipliers, 136 indicates a weight control part, 137, 138 indicate adders, 139 indicates a channel estimation part, 140, 141 indicate multipliers, 142 indicates a combining part (Σ), and 143 indicates a decision part.
The despreading parts 134-1-134-m in each of the despread processing parts 133 are provided to the respective array antenna elements 131-1-131-m. Each of the despreading parts 134 despread the received signal by using a spreading code supplied from a despreading code generator (not shown). The output signals of the despreading parts 134-1-134-m are respectively applied to both the multipliers 135-1-135-m and to the weight control part 136. The weight control part 136 calculates weighting factors based on the output signals of the neighboring despreading parts 134-1-134-m and the output signal of the adder 138.
The weighting factors (complex numbers) have values depending on the directions in which the radio wave comes to the array antenna elements 131-1-131-m. The multipliers 135-1-135-m multiply the weighting factors by the despread output signals. The resultant output signals of the multipliers 135-1-135-m are in phase with each other and are then applied to the adder 137. The above adding process is known as a beam forming process. The channel estimation part 139 then produces a channel estimination value similar to the despread processing parts 111 of the interference replicating unit 103 of FIG. 11. The multiplier 140 multiplies the output signal of the adder 137 (despread output signal) and the channel estimation value (complex conjugate). The resultant output signals of the despread processing parts 133 are then applied to the combining part 142.
The combined output signal from combining part 142 is input to the decision part 143, where it is compared with a threshold value for deciding data. The decision output signal from the decision part 142 is fed back to the multiplier to be multiplied by the channel estimation value so that a signal corresponding to the output signal of the adder 137 is obtained. The adder 138 calculates the difference between the output signal of the multiplier 141 and the output signal of the adder 137. The difference thus calculated is input to the weight control part 136, which produces the weighting factors having values that achieve optimal synthesizing in the adder 137.
FIG. 13 shows a conventional RAKE receiver device, in which a reference number 151 indicates an antenna and 152 indicate finger parts. Reference number 153 indicates a searcher, 154 indicates a combining part, 155 indicates a decision part, 156 indicates a despreading part, 157 indicates a multiplier, 158 indicates a spreading code output part, 159 indicates a dump filter, 160 indicates a delay adjustment part (τ), 161 indicates a channel estimation part, 162 indicates a multiplier, 163 indicates matching filter, 164 indicates an averaging part, 165 indicates a memory (RAM) for storing a delay profile, and 166 indicates a path detection part which performs a finger allocation.
The RAKE receiver device allocates the finger parts 152 to the different paths in accordance with the delay profile obtained by the searcher 153. The averaging part 164 of the searcher 153 averages the output signal from the matching filter 163. The delay profile thus obtained is stored in the memory 165. The path detection part 166 performs a path decision using the delay profile. For example, if a delay profile P shown in FIG. 14 is obtained, the path detection part 166 compares the receive level with a threshold value TH1 and detects paths P1, P2 and P3 that exceed the threshold value TH1. The path detection part 166 then allocates the paths P1, P2 and P3 to the first, second and third finger parts 152, respectively.
The above allocation is attained by adjusting the delay times of the finger parts 152 in accordance with the phase differences among the paths P1, P2 and P3. Then a start signals is applied to the spreading code output part 158 at the timings corresponding to the paths P1, P2 and P3. Hence, the despreading process for the received signals is started. Therefore, the received signals obtained through the paths P1, P2 and P3 are synthesized so that the receive sensitivity is improved.
As has been described previously, the radio communication device of the DS-CDMA system is equipped with the interference canceller for eliminating interference from another channel, and thus improves the receive characteristics. Further, the radio communication device of the DS-CDMA system uses the array antenna and the adaptive array receiver that synthesizes the received signals on the basis of the directions in which the radio waves arrive at the array antenna elements.
However, a mere application of the array antenna to the DS-CDMA system to form the adaptive array receiver device makes the structure very complex and thus increases the cost of the system. Particularly, the complex structure also results from the unique arrangement of the DS-CDMA system in which radio communications are performed in a state in which high-rate channels of high transmission rates and low-rate channels of low transmission rates are mixed.